1. Field of the Invention
The present invention relates to a method and means for laying a marine pipeline and more particularly, but not by way of limitation, to a variable departure reel and straightening mechanism for feeding a continuous length of pipe to a trenching plow having a retractable plowshare and backfilling apparatus, the plow being instrumented to insure the proper position of the plow with respect to the natural curve of the pipe entering the trench.
2. History of the Prior Art
Oil and gas marine pipelines are commonly buried in trenches to protect them from storm currents, anchor and a variety of other material hazards.
Such pipelines are commonly constructed of steel pipe by one of two methods. One method is to weld standard mill lengths of pipe continuously together aboard a barge and move the barge out from under the pipeline. The other method is to weld the pipe at a shore facility and then coil it onto a barge mounted reel. This causes the pipe metal to yield in bending as it conforms to the reel surface. Therefore, the pipe must be yielded in the opposite direction or straightened as it is uncoiled from the reel for laying on the ocean floor.
The commonly constructed reel system is mounted aboard a barge with the reel axis either vertical or horizontal. The straightening is accomplished by an arrangement of either two or three rollers which are configured to continuously stress the pipe in bending the opposite direction within the plane of the coil thus effecting a continuous straightening of said pipe. The straightening rollers are typically located distant from the reel so that the pipe will enter said rollers at a reasonably constant angle which is preferred to be normal to the roller axis and in a plane parallel to the axis of the rollers.
The reason for this distance requirement is that the pipe comes off the reel from constantly varying points across the width of the reel and also comes off said reel at varying tangent positions because the pipe is coiled onto the reel in multiple layers. A typical system is set forth in the patent to Nicholson et al, U.S. Pat. No. 3,872,680, issued Mar. 25, 1975 for a "Method and Apparatus For Laying Pipelines".
Because of this arrangement, the pipe attitude in relation to the barge is normally substantially parallel to the deck as it is played over the stern. Because of this pipe attitude, it is necessary that the pipe be carefully maintained in an S-shaped curve as it is deployed to the sea floor. The bend at the stern of the barge is commonly referred to as the overbend and the bend at the sea floor is commonly referred to as the sagbend. Both of these bends must be carefully maintained at a radius large enough to keep the pipe metal within its elastic stress range in order to prevent buckling.
One way to accomplish this is to employ a ramp, commonly referred to as a stinger, as shown in said Nicholson patent (supra), which combines the use of the stinger and the application of tension to the pipeline in order to prevent buckling at the overbend or at the sagbend. The stinger causes many problems in that it is subject to current drag and loads imposed by the barge movement in the seaway. This stinger may be used with or without a diverter apparatus which provides a plurality of rollers in order to orient the pipe as it leaves the barge at a desired departure angle. Further, when the departure angle is completely controlled from the stern of the barge, rough seas will cause the departure angle to attempt to vary thereby creating the danger of buckling during the overbend operation.
The aforementioned patent to Nicholson et al teaches at FIG. 6 the locating of a straightening apparatus and the reel near the stern of the barge which would permit the pipe to enter the water in a substantially tangent orientation with respect to the reel. This would be a desirable operation if it were not for the continuous adjustments that would have to be made to the straightening apparatus, which is mounted on a pedestal just aft of the reel. Hence, the existing use of pipe reels in the laying of marine pipelines is plagued with many disadvantages.
Substantially all of the prior art plows employed in trenching marine pipelines have been adaptations of conventional agricultural plows mounted in a sled-type frame. In as much as agricultural plows are primarily intended to turn soil over, their usage in digging deep trenches is inefficient and requires large pulling forces. The typical agricultural plowshare normally has a V-type configuration for diverting the soil to the sides as it is being pulled across the ocean floor. However, this continuous movement of the soil to the sides causes extensive compressive loads on the trench walls thereby increasing the drag of the plowshare.
Further, marine oil and gas pipelines are not buried at a constant depth. For example, next to a well platform, the trench depth is commonly six feet while a distance of 600 feet or more from the platform the trench depth may be four feet. Substantially all of the plows being used in marine pipeline activities are not adjustable by depth.
Further, in offshore pipeline oil fields, there are many existing pipelines that must be crossed by a new construction pipeline. These pipelines must be located prior to beginning construction because the trenching plow would inflict serious damage on any pipeline it encounters. A major detraction to a plow trenching method is that at every foreign pipeline crossing, the laying process must be interrupted. The pipeline must be capped and laid on the ocean floor, then the lay barge must be positioned over the plow to pick it up and reset it past the foreign pipeline. This operation involves considerable time and difficulty.
The patent to Suzuki et al, U.S. Pat. No. 4,011,727, issued Mar. 15, 1977 for a "Movable Cable Plow For Constructing Underwater Cable", teaches a vertically adjustable plowshare. However, upon vertical adjustment, the plowshare and its associated cable guide means both must be lifted since the cable guide means rides in the trench behind the plowshares.
Further, since the plowshare of the Suzuki et al cable laying device is elevated by rotation about a forward pivot pin, the point, even when lifted out of contact with the ocean floor, is set at an angle which could damage pipelines or cables laying on the surface.
Offshore oil and gas pipelines are commonly left in open trenches which are eventually backfilled by movement of the bottom soil due to ocean currents. However, in areas where there is little or no current, the line often remains uncovered for years.
There is a backfilling apparatus taught in the patent to Sherrod, U.S. Pat. No. 3,347,054, issued Oct. 17, 1967 for an "Underwater Pipe Trenching Device". However, a plate at the rear of the closing blades causes the soil to build up thereby increasing the drag and also scraping the trench surface flat whereupon subsequent settling leaves the pipe inadequately covered.